This study intends to investigate radiation-induced humoral factors and their effects on chromosomal integrity in lymphocytes of rats. Chromosomal damage following radiation is well known. Recent evidence has suggested, however, that some chromosomal damage may be mediated by radiation induced plasma factors. It is the objective of this study to investigate radiation-dependent mutagenic humoral factors after in vivo and in vitro radiation. This knowledge might provide new avenues for the prevention of deleterious somatic or genetic effects and, in addition, a better basis for the therapeutic use of radiation.